1. Field of the Invention
The present invention relates to a method of measuring the thicknesses of each layer coated on the surface of the sheet material, and more paticularly relates to a method of measuring respectively the thickness of each layer coated on both surfaces of the sheet material and an apparatus for performing the method.
2. Description of the Prior Art
A duplex laser thickness meter is known which utilizes a laser displacement meter and the like, as an apparatus for measuring the thickness of a layer coated on the base material, such as adhesive tapes, magnetic tapes, electrode substrate of a battery and the like.
FIG. 1 shows an apparatus for measuring the thickness of a layer coated on the base material with a duplex laser thickness meter utilizing a laser displacement meter. This apparatus has a pair of displacement meters 31 and 33 arranged opposite to each other, to measure the distance between the one displacement meter 31 and the one surface of the coated sheet S with said displacement meter 31, and to measure the distance between the other displacement meter 33 and the other surface of the coated sheet S with said displacement meter 33.
These displacement meters 31 and 33 are devices to measure simultaneously each distance described above at the same position with each other with regard to the widthwise direction of the coated sheet S. And the distance to be measured directly by each displacement meter 31, 33 is the distance from the immovable reference position determined by the arranged position of the displacement meters 31 and 33 to the surface of the coated sheet S. Since the alienated distance D between the displacement meters 31 and 33 is a known value, assuming that the measured value by the displacement meter 31 is Da (distance between the displacement meter 31 and the one surface of the coated sheet 5), and the measured value by the displacement meter 33 is Db (distance between the displacement meter 33 and the other surface of the coated sheet S), the whole thickness T of the coated sheet S is calculated from the equation T =D-Da-Db with the measurement at the P-point in FIG. 1A, and the thickness Tc of the base material sheet of the coated sheet S is obtained with the measurement at the Q-point in FIG. 1B, respectively.
Here, if the position in the vertical direction of the coated sheet S shown in FIG. 1 is not changed, that is, if the sheet S Is not oscilated, at the time of measurement at P-point and at the time of measurement at Q-point, the thickness Ta of the layer Sa coated on the one surface of the sheet material Sc is calculated from the difference of the values Da measured by the displacement meter 31 at the time of measurement at P-point and at the time of measurement at Q-point, and the thickness Tb of the layer Sb coated on the other surface of the sheet material Sc is calculated from the difference of the values Db measured by the displacement meter 33 at the time of measurement at P-point and at the time of measurement at Q-point.
In the general coating process, however, since there is a drying process having a long free span, it is unavoidable to oscillate the coated sheet S, thereby it is unavoidable to cause an error in the thickness measurement of the layer.
As shown in FIG. 2A, at the time of the measurement at P-point, the coated sheet S is in the middle position between the displacement meters 31 and 33, but at the time of the measurement at Q-point, as shown in FIG. 2B, if the coated sheet S is displaced by .DELTA.y to the side of the displacement meter 31, the thickness Ta of the layer Sa is measured to be smaller than the true value by .DELTA.y, and the thickness Tb of the layer Sb is measured to be larger than the true value by .DELTA.y.
The thickness of each layer is determined as (measured value on the surface of the base material)--(measured value on the surface of the layer). But, though the measured value on the surface of the base material is always changed due to the oscillation of the coated sheet, the measurement on the surface of the layer (measurement at P-point) and the measurement on the surface of the base material (measurement at Q-point) are measured with a time difference, therefore the measurement error is always caused.